1. Field of the Invention
Aspects of the present invention relate to a fuel cell system, and more particularly, to a fuel cell system that promptly increases the temperature of a fuel cell stack during a start up operation of the fuel cell system and a method of managing the fuel cell system.
2. Description of the Related Art
Conventionally, a fuel cell is an apparatus that directly converts chemical energy of a fuel into electric energy through a chemical reaction. In particular, a fuel cell is used as a power generator and generates electricity as long as fuel is supplied to the fuel cell. FIG. 1 illustrates the energy conversion structure of a unit cell 10 of a conventional fuel cell. As illustrated in FIG. 1, when air, containing oxygen, is supplied to a cathode 1, and a fuel that includes hydrogen is supplied to an anode 3, an inverse water-electrolysis reaction is performed through an electrolyte layer 2, thereby generating electricity. However, the electricity generated by a unit fuel cell 10 does not have a high enough voltage for practical use. Therefore, unit fuel cells 10 are typically arranged in series in of the form of a stack 100 (FIG. 2). Each of the fuel cells 10 in the stack 100 includes the electrolyte layer 2, and the cathode 1 and the anode 3 respectively disposed on each side of the electrolyte layer 2. Flow path plates 4 having surface flow paths 4a, for supplying oxygen or hydrogen gas to the cathode 1 and anode 3, are installed adjacent to the unit fuel cells 10. Accordingly, when hydrogen and oxygen are supplied to the stack 100 as illustrated in FIGS. 2 and 3, oxygen or hydrogen respectively passes through the cathode 1 or the anode 3 through the flow paths 4a of each of the unit fuel cells 10 and circulates. Since heat is also generated by the electrochemical reactions of the fuel cell system, the heat needs to be removed in order to operate the fuel cell system within a normal operating temperature range. To this end, a cooling plate 5, which transports cooling water that absorbs heat, is installed in the fuel cell stack at an interval of about one cooling plate 5 for about every five or six unit fuel cells 10. Accordingly, cooling water passes through flow paths 5a of the cooling plate 5 and absorbs the heat generated by the stack 100. The cooling water, which absorbs the heat, is cooled by secondary cooling water in a heat-exchanger H5 (see FIG. 5) and is circulated again in the stack 100.
In previous fuel cell systems, secondary cooling water, after heat-exchanging with the cooling water circulating in the stack 100, was sent directly to the hot water tank 120 in order to be used as hot water for extraneous purposes. In other words, the fuel cell system could also function as a hot water heater. However, the temperature of the secondary cooling water was generally not sufficient for the secondary cooling water to be very useful as hot water. Therefore, recently, a process burner 110 has been installed to use up the remaining hydrogen in a fuel cell system. A process burner 110 operates by using any hydrogen that was not consumed in the stack 100 as a main fuel. Water is heated using the process burner 110 and is stored in a hot water tank 120.
The fuel source that supplies hydrogen to the stack 100 may be a hydrocarbon-based material, such as natural gas. As illustrated in FIG. 4, hydrogen is generated from a fuel source in a fuel processing unit 200 and is supplied to the stack 100. The fuel processing unit 200 includes a desulfurizer 210, a reformer 220, a burner 230, a water supply pump 260, first and second heat-exchangers H1 and H2, and a carbon monoxide (CO) removing unit including a CO shifter 251 and a CO remover 252. Hydrocarbon based gas, which flows from a fuel tank 270, and water vapor, which comes from a water tank 280 connected to a water supply pump 260, react in the reformer 220, which is heated by the burner 230, and thereby, generating hydrogen in the reformer 220. Carbon dioxide and CO are generated as by-products. At this point, the generated CO must be removed, because if a fuel mixed with more than 10 ppm of CO, is supplied to the stack 100, the anode 3 becomes poisoned and the performance of the fuel cell 10 rapidly decreases. Accordingly, the CO shifter 251 and the CO remover 252 are disposed at the exit of the reformer 220 to control the amount of CO to be 10 ppm or less. In the CO shifter 251, CO and water vapor react to produce carbon dioxide, and in the CO remover 252, CO is directly oxidized by oxygen. The fuel passing through the CO shifter 251 includes CO with an amount of 5000 ppm or less, and when the fuel exits the CO remover 252, the amount of CO in the fuel is 10 ppm or less. The desulfurizer 210 removes sulfur contaminants from the fuel source before the fuel source enters the reformer 220. Sulfur compounds can poison the anode 3 even in amounts as small as 10 parts per billion (ppb), and also can poison catalysts used in the CO shifter 251 and CO remover 252. Therefore, sulfur compounds are absorbed and removed from the fuel source by passing the fuel source through the desulfurizer 210.
Accordingly, in operating a fuel cell system including the fuel processing unit 200 and the stack 100, hydrogen is generated in the above described manner in the fuel processing unit 200, and electricity is generated in the stack 100 using hydrogen supplied from the fuel processing unit 200 as a fuel. As illustrated in FIGS. 2, 3 and 4, hydrogen passes through a hydrogen flow path to contact the anode 3, and air, which is the oxygen source, passes through an oxygen flow path to contact the cathode 1.
The temperature inside the stack 100 should be maintained at a constant temperature in order to properly operate the fuel cell system so that electricity can be constantly generated in the stack 100. For example, a stack may have a normal operating temperature of about 120° C. However, if a stack has been idle and has cooled to room temperature, it takes a long time to get back up to the normal operating temperature inside the stack 100. During a start up operation of the fuel cell system, in order to increase the temperature of the stack 100, the cooling water reservoir 130 is heated using an electric heater, and the heated cooling water is circulated to the stack 100, thereby increasing the temperature of the stack 100. The temperature of the stack 100 is also increased by heat radiation while electricity is generated in the stack. However, at the start of the operation of the fuel cell system, such as when the stack is at or near the room temperature, it may take almost an hour until the temperature of the stack 100 reaches a temperature at which the fuel cell system is running properly, such as, for example, 120° C. Typically, the start up of the fuel processing unit 200 is much faster, and the fuel processing unit is able to supply hydrogen much sooner than the stack 100 is able to use the hydrogen efficiently, since, the fuel cell system cannot operate efficiently until the temperature of the stack 100 reaches the predetermined temperature. Thus, a large delay occurs while waiting for the temperature of the stack 100 to reach the predetermined temperature.
Therefore, to easily operate the fuel cell system, a method of increasing the temperature of the stack 100 more rapidly during a start up operation of the fuel cell system is desirable.